Christian Demigné

Quercetin is recovered in human plasma as conjugated derivatives which retain antioxidant properties

Quercetin is one of the most abundant flavonoids in the human diet. This study aimed to determine the plasma concentrations of quercetin in 10 healthy volunteers after the consumption of a complex meal rich in plant products. Quercetin was determined in plasma (2 h before, and 3, 7 and 20 h after the meal), and in a duplicated portion of the meal by HPLC analysis with an electrochemical detection. The amount of ingested quercetin was estimated to be 87 mg. Before the meal, quercetin concentration in hydrolyzed plasmas ranged from 28 to 142 nM. A marked increase was observed 3 h after the meal in all subjects, with a mean concentration of 373 nM (S.E.M.=61). After 7 h, quercetin concentration in hydrolyzed plasmas decreased and after 20 h basal levels were found again. The antioxidant capacities of quercetin, 3′‐O‐methylquercetin, and of some of their conjugated derivatives were compared by the measurement of the conjugated dienes resulting from the Cu2+‐induced oxidation of human LDL. 3′‐O‐Methylquercetin and conjugated derivatives of quercetin significantly prolonged the lag phase, but the magnitude of their effect was about half that of the aglycone.

Bioavailability of rutin and quercetin in rats

Quercetin is a powerful antioxidant which is widely distributed in edible plants, mainly as glycosides such as rutin. It has been reported to be absorbed in mammals, but its metabolism needs further investigation to evaluate its possible physiological effects. We compared the evolution of the absorption of quercetin and rutin in rats fed with supplemented diets. Rutin was absorbed more slowly than quercetin because it must be hydrolysed by the cecal microflora, whereas quercetin was absorbed from the small intestine. Conjugated derivatives of quercetin, and its methylated forms isorhamnetin and tamarixetin, were recovered in plasma from rats receiving the two kinds of experimental diets after the first meal, but after 10 days, no traces of tamarixetin were detected anymore. The rate of elimination of quercetin metabolites seems very low, and high plasma concentrations are easily maintained with a regular supply of quercetin or rutin in the diet.

Effect of propionate on fatty acid and cholesterol synthesis and on acetate metabolism in isolated rat hepatocytes

In the present study the actual role of propionic acid in the control of fatty acid and cholesterol synthesis was investigated in isolated liver cells from fed rats maintained in the presence of near-physiological concentrations of glucose, glutamine and acetate. Using 3H2O for lipid labelling, propionate appears as an effective inhibitor of fatty acid synthesis and to a lesser extent of cholesterol synthesis, even at the lowest concentration used (0.6 mmol/l). Butyrate is a potent activator of both synthetic pathways, and the activating effect was not counteracted by propionate. Using 1-[14C]acetate, it was observed that propionate at a moderate concentration, or 1 mmol oleate/l, are both very effective inhibitors of 14C incorporation into fatty acid and cholesterol. This incorporation was drastically inhibited when propionate and oleate were present together in the incubation medium. The net utilization of acetate by rat hepatocytes was impaired by propionate, in contrast to oleate. 1-[14C]butyrate was utilized at a high rate for fatty acid synthesis, but to a lesser extent for cholesterol synthesis; both processes were unaffected by propionate. Intracellular citrate concentration was not markedly depressed by propionate, whereas it was strongly elevated by butyrate. In conclusion, propionate may represent an effective inhibitor of lipid synthesis when acetate is a major source of acetyl-CoA, a situation which is encountered with diets rich in readily-fermentable fibres. The present findings also suggest that propionate may be effective at concentrations close to values measured in vivo in the portal vein.

Effects of two fermentable carbohydrates (inulin and resistant starch) and their combination on calcium and magnesium balance in rats

Resistant starch and inulin are complex carbohydrates that are fermented by the microflora and known to increase colonic absorption of minerals in animals. The fermentation of these substrates in the large bowel to short-chain fatty acids is the main reason for this increase in mineral absorption. The purpose of the present study was to examine the potential synergistic effect of a combination of these two fermentable carbohydrates. For this purpose, thirty-two adult male Wistar rats weighing 200 g were used in the present study. The rats were distributed into four groups, and fed for 21 d a fibre-free basal purified diet or diet containing 100 g inulin, or 150 g resistant starch (raw potato starch)/kg diet or a blend of 50 g inulin and 75 g resistant starch/kg diet. After an adaptation period of 14 d, the rats were then transferred to metabolic cages and dietary intake, faeces and urine were monitored for 5 d. The animals were then anaesthetized and caecal Ca and Mg absorption were measured. Finally, the rats were killed and blood, caecum and tissues were sampled. Ca and Mg levels were assessed in diets, faeces, urine, caecum and plasma by atomic absorption spectrometry. Our results confirmed that inulin and resistant starch ingestion led to considerable caecal fermentation in the three experimental groups compared with the control group diet. Moreover, both carbohydrates significantly increased the intestinal absorption and balance of Ca and Mg, without altering the plasma level of these two minerals. Interestingly, the combination of the studied carbohydrates increased significantly the caecal soluble Ca and Mg concentrations, the apparent intestinal absorption and balance of Ca, and non-significantly the plasma Mg level. In conclusion, a combination of different carbohydrates showed synergistic effects on intestinal Ca absorption and balance in rats. Further studies with other types of carbohydrate combinations should be carried out to extend these findings.

Bioavailability, metabolism and physiological impact of 4-oxo-flavonoids

4-oxo-flavonoids are dietary compounds widely distributed in the plant kingdom. Interest in these substances has arisen because of possible effects on human health. Indeed, in spite of extensive metabolism by the intestinal flora, 4-oxo-flavonoids seem to be sufficiently absorbed to have physiological effects. This review discusses the possibilities of a positive role in cardiovascular disease and cancer prevention, particularly by their antioxidant properties and their ability to modulate the activity of many enzymes, and summarizes the data concerning an eventual carcinogenicity of 4-oxo-flavonoids.

Acidic fermentation in the caecum increases absorption of calcium and magnesium in the large intestine of the rat

The effect of fermentation on colonic absorption of Ca and Mg was investigated in 8-week-old rats adapted to diets containing either digestible wheat starch (DS diets) or including resistant starch, i.e. 350 g raw potato starch/kg (RS diets). The dietary Ca level of the DS and RS diets was 2.5 or 7.5 g/kg. RS diets resulted in enlargements of the caecum together with hypertrophy of the caecal wall. Acidification of the caecal contents by microbial fermentation of RS was influenced by the dietary Ca level. Very acidic pH conditions and relatively low concentrations of short-chain fatty acids, in the presence of lactic acid fermentation, were observed with the 2.5 g Ca/kg level. Rats fed on RS diets had a higher percentage of soluble Ca (and inorganic phosphate) in the caecum, particularly of rats adapted to the high Ca level. As a result of the hypertrophy of the caecal wall and of an elevated concentration of soluble Ca, the caecal absorption of Ca was 5-6-fold higher in the RS groups than in the DS groups. The difference between dietary intake and faecal excretion (DI-FE) of Ca was higher in rats fed on RS diets than in those fed on DS diets, when the dietary Ca level was 2.5 g/kg. With the higher Ca intake the elevated rate of Ca absorption from the caecum in RS-fed rats was not paralleled by an enhanced DI-EE difference: this suggests a shift of the Ca absorption towards the large intestine. Feeding RS diets also enhanced Mg caecal absorption, resulting in a substantially higher DI-FE difference for Mg, especially with the 2.5 g Ca/kg diets, because a high Ca intake tends to inhibit Mg absorption. The present findings support the view that the large intestine may represent a major site of Ca (and Mg) absorption when acidic fermentations take place. This process could improve the digestive Ca balance when the dietary Ca supply is low; when the Ca supply is affluent, it rather shifts Ca absorption towards a more distal site of the digestive tract.

Part of quercetin absorbed in the small intestine is conjugated and further secreted in the intestinal lumen

Rutin and quercetin absorption and metabolism were investigated in rats after in situ perfusion of jejunum plus ileum (15 nmol/min). In contrast to rutin, a high proportion of quercetin (two-thirds) disappeared during perfusion, reflecting extensive transfer into the intestinal wall. Net quercetin absorption was not complete (2.1 nmol/min), inasmuch as 52% were reexcreted in the lumen as conjugated derivatives (7.7 nmol/min). Enterohepatic recycling contribution of flavonoids was excluded by catheterization of the biliary duct before perfusion. After a 30-min perfusion period, 0.71 microM of quercetin equivalents were detected in plasma, reflecting a significant absorption from the small intestine. The differential hydrolysis of effluent samples by glucuronidase and/or sulfatase indicates that the conjugated forms released in the lumen were 1) glucuronidated derivatives of quercetin and of its methoxylated forms (64%) and 2) sulfated form of quercetin (36%). In vitro quercetin glucuronides synthetized using jejunal and ileal microsomal fractions were similar to those recovered in the effluent of perfusion. These data suggest that glucuronidation and sulfatation take place in intestinal cells, whereas no glucurono-sulfoconjugates could be detected in the effluent. The present work shows that a rapid quercetin absorption in the small intestine is very effective together with its active conjugation in intestinal cells.

Fructooligosaccharides enhance mineral apparent absorption and counteract the deleterious effects of phytic acid on mineral homeostasis in rats

Phytic acid (PA) and fructooligosaccharides (FOS) such as inulin are two food components that are able to modify mineral absorption negatively or positively. The influence of PA and FOS on the cecal and apparent mineral absorption as well as on the mineral status (plasma, hepatic, and bone) were investigated in four groups of rats fed one of the experimental diets: a fiber-free (FF) diet, a FF diet containing 7 g/kg PA (FF + PA), a diet containing 100 g/kg inulin (FOS), or a FOS diet containing 7 g/kg PA (FOS + PA). The cecal enlargement together with the acidification of cecal pH in rats adapted to FOS diets led to an improved Ca and Mg cecal absorption. Mineral apparent absorption was significantly enhanced by FOS ingestion (Ca, +20%; Mg, +50%; Fe, +23%; Cu, +45%), whereas PA decreased this factor only for trace elements (Fe, -48%; Zn, -62%; Cu, -31%). These inhibitory effects of a FF + PA diet have repercussions on blood (Mg, -15%; Fe, -12%; transferrin saturation -31%), liver (Mg, -18%; Fe, -42%; Zn, -25%), and bone (Zn, -25%) variables. However, the introduction of FOS into a PA diet counteracted these observed deleterious effects by stimulating bacterial hydrolysis of PA (+60% in rats adapted to FOS + PA compared to those fed the FF + PA diet) and by improving cecal absorption of minerals.

Comparison of the bioavailability of quercetin and catechin in rats.

Quercetin and catechin are present in noticeable amounts in human diet and these polyphenolic compounds are supposed to exert beneficial effects on human health. However, their metabolic fates in the organism have never been compared. In the present study, rats were fed a 0.25% quercetin or a 0.25% catechin diet. Quercetin and catechin metabolites were analyzed in plasma and liver samples by high-performance liquid chromatography coupled to an ultraviolet or a multielectrode coulometric detection. All plasma metabolites were present as conjugated forms, but catechin metabolites were mainly constituted by glucuronidated derivatives, whereas quercetin metabolites were sulfo- and glucurono-sulfo conjugates. Quercetin was more intensively methylated than catechin in plasma. The plasma quercetin metabolites are well maintained during the postabsorptive period (approximately 50 microM), whereas the concentration of catechin metabolites dropped dramatically between 12- and 24-h after an experimental meal (from 38.0 to 4.5 microM). In the liver, the concentrations of quercetin and catechin derivatives were lower than in plasma, and no accumulation was observed when the rats were adapted for 14 d to the supplemented diets. The hepatic metabolites were intensively methylated (90-95%), but in contrast to plasma, some free aglycones could be detected. Thus, it clearly appears that studies dealing with the biological impact of these polyphenols should take into account the feature of their bioavailability, particularly the fact that their circulating metabolites are conjugated derivatives.

Resistant starch is more effective than cholestyramine as a lipid-lowering agent in the rat.

Amylase-resistant starch (RS) represents a substrate for the bacterial flora of the colon, and the question arises as whether RS shares with soluble fibers common mechanisms for their lipid-lowering effects. It is uncertain whether a cholesterol-lowering effect depends basically on an enhanced rate of steroid excretion or whether colonic fermentations also play a role in this effect. In the present study, the effect of RS (25% raw potato starch), of a steroid sequestrant (0.8% cholestyramine), or both were compared on bile acid excretion and lipid metabolism in rats fed semipurified diets. RS diets led to a marked rise in cecal size and the cecal pool of short-chain fatty acids (SCFA), as well as SCFA absorption; cholestyramine did not noticeably affect cecal fermentation. Whereas cholestyramine was particularly effective at enhancing bile acid excretion, RS was more effective in lowering plasma cholesterol (−32%) and triglycerides (−29%). The activity of 3-hydroxy-3-methylglutaryl-CoA reductase was increased fivefold by cholestyramine and twofold by RS. This induction in rats fed RS diets was concomittant to a depressed fatty acid synthase activity. In rats fed the RS diet, there was a lower concentration of cholesterol in all lipoprotein fractions, especially the (d=1.040−1.080) fraction high-density lipoprotein (HDL1), while those fed cholestyramine had only a significant reduction of HDL1 cholesterol. In contrast to cholestyramine, RS also depressed the concentration of triglycerides in the triglyceride-rich lipoprotein fraction. There was no noticeable synergy between the effects of RS and cholestyramine when both were present in the diet. This suggests that the cholesterol-lowering effect of RS is not limited to its capacity to enhance bile acids excretion. The difference between RS and cholestyramine could relate to the capacity of fermentation end-products to counteract the upregulation of cholesterol and bile acid biosynthesis. Thus, in the absence of fermentation in the large intestine, a high rate of bile acids excretion is not always sufficient to elicit a cholesterol-lowering effect.